Oceans and climate change: physical processes

As part of our oceans and climate change series, this article explores physical ocean processes and the ways that they interact with, and respond to, climate drivers. We examine key issues such as changes in ocean circulation and sea level rise. Part 1 of our oceans series can be found here. Written in collaboration with Sir Alister Hardy Foundation for Ocean Science, their original reports and outreach materials can be accessed here.

Changes to temperature, circulation and ice cover

The North Atlantic Ocean is the driver of Earth’s ocean circulation. This is due to the heat transfer of the North Atlantic currents – the warm North Atlantic current (which keeps Europe temperate) heads northwards and reaches the Arctic where it cools, becomes denser (heavier) and sinks. It then returns south along the eastern coast of North America towards the equator as a cooler ocean current. This Atlantic Meridional Overturning circulation (AMOC) is what is more commonly termed the ‘ocean conveyor’.

Research shows that rising global temperatures are melting the Greenland ice sheet, Arctic ice caps, permafrost, and sea ice. This means that freshwater inputs into the Arctic Ocean (and ultimately the North Atlantic) are changing the temperature, direction, and strength of the currents.

Although datasets are limited, because longer-term monitoring is needed, the AMOC shows large variability – even on daily timescales. There is no evidence at present for any long-term reorganisation of the AMOC. However, it is clear that sea-ice in the Arctic has shown pronounced changes in coverage and thickness over the last 30 years. On average, the extent of sea ice has declined by 11% during this time, with evidence of a recent acceleration. Between 1980 and 2008 (28 years) the thickness of sea ice reduced by 50%, to 1.75 m.

Warming in the Arctic is by far the fastest on Earth and has occurred at twice the global average rate. This is due to amplification from feedback mechanisms such as the albedo effect.

In contrast, in 2014, Antarctic sea ice extent reached a new record high since satellite records began. The rate of increase of May Antarctic sea ice extent is 2.88% per decade (1979 to 2015). Over the same period, in the Arctic, there is a decrease in ice extent at a rate of 2.33% per decade. These differences are due to localised factors, and indicate that present day climate change will not necessarily have the same impacts everywhere across the globe. You can read more about changes in Arctic and Antarctic sea ice at NASA, NOAA, and NSIDC.

The Arctic Ocean surrounding Greenland is projected to warm particularly rapidly, by 2.8-7.8°C by 2100. With this warming it is estimated that it may be free of summer sea-ice within 20 to 30 years. This rate and scale of warming is comparable to other warming events that have occurred previously in the Holocene. If we can understand these past changes, through using the palaeo record, we can start to develop more robust predictions for future ocean change.

In European seas, surface and deep water temperatures have increased markedly over the last few decades. The Baltic Sea is set to increase in temperature by around 2-4°C by 2100, the North Sea is predicted to warm by 0.8°C by 2040, and the Mediterranean is predicted to become more saline.

Key areas for future research on the impacts of climate change on ocean circulation include:

To continue with long-term monitoring programmes to improve our understanding of ocean-atmosphere interactions.

To continue to enhance and extend data collection so that we can build more reliable models of climate change and its impacts on the ocean.

Sea level and coastal erosion

Around 41% of the world’s population lives within 100 km of the coast.

Many of these people live within coastal cities vulnerable to sea-level rise.

Monitoring programmes and palaeo data indicate that sea level is now higher than at any other time in the last 2,000 years.Satellite measurements show a rapid increase in global sea level since around 1993, and levels are now rising at around 3.1mm per year. This rate is faster than the IPCC scenarios for 2100, and are almost double the average for the 20th century. These rates of change are spatially variable – they are not the same across the globe. In Europe alone, rates of sea level rise range from 1.8 to 3.6 mm per year. These spatial differences are due to local factors (such as tectonics). It is important to differentiate absolute and relative sea level change:

Absolute sea level is the surface height of the ocean relative to the centre of the Earth.

Relative sea levelis the height of the sea relative to nearby land – some land is rising (uplifting), some is static, and some is sinking (subsiding).

Why might sea level rise?

The rapid acceleration in sea level is mainly due to the thermal expansion of seawater and a large increase in volume caused by melting ice, though other factors also contribute to changes in sea level:

Expansion of seawater as it warms

Melting of land ice

Melting of the Greenland and Antarctic ice sheets

Gravity – like the moon, ice sheets have a gravitational attraction. As the Greenland ice sheet melts, the gravity change reduces sea levels in Northwest Europe while melting of the West Antarctic ice sheet would have the opposite effect.

With these factors combined, estimates suggest that global sea level will rise by at least 1 m by 2100. The absolute and relative sea level will vary spatially due to variations in land subsidence or uplift.

Global mean sea level (deviation from the 1980-1999 mean) in the past and as projected for the future. For the period before 1870 (shown by the grey line) global measurements of sea level are not available and values are estimated. The red line is based on global mean sea level measurements from tide gauges. The green line shows global mean sea level observed from satellite altimetry. The blue shading represents the range of model projections for the 21st century, relative to the 1980 to 1999 mean. Beyond 2100, the projections are increasingly dependent on the emissions scenario. The thickness of all lines represents the error associated with the past estimates, present day monitoring, and future modelling. Source: https://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-5-1-figure-1.html

Another important factor to consider when assessing the impacts of sea level on coastal communities is the frequency and magnitude of storm surge events. With current data, researchers have not identified a clear long-term link between climate change and storm surge activity. This is because storm surges are extreme and rare events – it would take a long time to monitor enough storms urges to draw any reliable conclusions about their patterns. What we do know is that storm surges present a serious risk to coastal communities. The impacts of climate change, and associated sea level changes, on storm surge dynamics is a key area for further research and continued long-term monitoring is needed.

It is expected that sea level rise will lead to: an increase in sea level rise and the rate of coastal erosion; greater impacts on coastal habitats; potential problems with built structures; and an increase in the magnitude and frequency of storm events – though these predictions are not yet supported by enough long-term evidence. It is also possible that climate change will lead to an increase in the transportation of contaminants (nutrients and pollutants) from the land to the sea – dry summers with intense downpours for example, may flush contaminants into river systems and eventually offshore.